The present invention relates to a method for packaging plastic material having a finite size and shape using a film to surround the material, and more particularly to a method for packaging hot melt adhesives using a coextrusion process, the resulting package formed thereby, and the film composition used therein.
Hot melt adhesives are substantially solid at room temperature, but are applied in a molten or flowable state. Typically, hot melt adhesives are supplied in the form of solid blocks, pillows or pellets. However, since hot melt pressure sensitive adhesives (HMPSA) or other plastic materials are typically sticky and/or soft at room temperature, there are problems associated with handling and packaging of HMPSA. Regardless of the form in which it is provided, a HMPSA not only sticks or adheres to hands, mechanical handling devices and to itself, but it also picks up dirt and other contaminates. In addition, the individual pieces of HMPSA and/or other low softening point plastic materials will tend to flow or block together into a single solid mass rendering such materials difficult to be handled and/or packaged. As a result, containment during shipment and/or storage periods is a critical aspect of delivering HMPSA, other soft adhesives, or any kind of plastic materials such as sticky and/or soft polymers.
To address this troublesome difficulty, those skilled in the art have turned to packaging techniques such as, for example, shipping adhesive compositions in rigid containers, such as 55 gallon drums, or silicone coated, corrugated or particleboard boxes, or alternatively, silicone coated polymer-based trays which are typically manufactured from a high density polyethylene. While these solutions will solve the problem, noted above, in some industrial environments, and applications, it presents a multiplicity of additional difficulties, and other drawbacks which have detracted from their usefulness. For example, the more traditional methods of shipping hot melt adhesive compositions create problems related to the cost and disposal of the packaging materials. In this regard, the silicone coated polymer based trays employed for shipping some types of hot melt adhesive compositions are relatively expensive to purchase, and the presence of a silicone coating makes such a container very difficult if not impossible to recycle. Therefore, these trays, or other similar containers, in many instances, are merely collected and disposed of in landfill sites, thereby creating a troublesome source of pollution. Further, these silicone coated containers create other difficulties in that under certain environmental circumstances a great deal of physical force must be applied to the adhesive to extract it from these containers, making the adhesive difficult and inconvenient to handle.
In response to the shortcomings noted above, some manufacturers have attempted to produce synthetic trays which are readily recyclable, or which can, alternatively, be returned and reused for subsequent shipments of adhesives. Recyclable containers, although attractive by their relative simplicity, have not been embraced widely in the industry due to an obvious lack of commercial appeal. More particularly, the development of a new, more readily recyclable tray has produced, as a byproduct, retooling, and other manufacturing expenses, which have been substantial in some respects, and which have worked as a deterrent to the adoption of this solution as an answer to the aforementioned shortcomings. Further, reusable containers are generally manufactured from a thicker gauge material to lend a measure of durability to the containers such that they may survive multiple uses. As a result, the per unit cost of such containers is usually significantly greater than disposable containers. Additionally, the costs attendant to collection and returning the spent containers or trays to the manufacturers for subsequent use has further caused this proposed solution to be quite expensive and therefore commercially unacceptable.
Many other different approaches have been tried to package plastic materials like HMPSA. One approach is to use a package material that is meltable together with and blendable into the molten adhesive composition itself just prior to application.
In one such method, molten adhesive is fed directly into a film formed in the shape of a tube, and then individual packaged adhesive units are formed by squeezing, sealing and cutting the tube. Such a process is sometimes referred to as a “form, fill and seal” process. For example, U.S. Pat. No. 5,373,682 describes a method where molten adhesive is pumped into a cylindrically-shaped tube of plastic film while the exterior surface of the film is sprayed with cooling water. The resultant individual packages referred to as “cartridges” are allowed to cool until the adhesive solidifies. Another example of making cartridges of this type is illustrated in U.S. Pat. No. 5,392,592.
In U.S. Pat. No. 6,430,898, low complex viscosity film materials are used to surround a hot melt adhesive. The film material is composed of an ethylene/α-olefin interpolymer, which is a metallocene or single-site catalyzed interpolymer of ethylene and at least one C3-C20α-olefin.
Rouyer et al, in U.S. Pat. No. 5,527,491, and U.S. Reissue Patent 36,177 approaches the problem of shipping cold flowing, pressure sensitive hot melt adhesive compositions by designing a package which encloses the pressure sensitive adhesive and is meltable along with the adhesive. In this instance, the package is a polymer based film which will melt when exposed to the heat of application machinery.
In PCT applications WO 02/061009 and WO 04/037671, it is claimed that low temperature adhesive application can be achieved when using low melting point (below 100° C.) polymers as packaging films, for example ethylene vinyl acetate (EVA) copolymers, ethylene methyl acrylate (EMA) copolymers, polyethylene copolymers, polypropylene copolymers or combinations thereof.
Yet another approach is to first form the adhesive into a finite size and shape, and then wrap the adhesive with a plastic film. For example, U.S. Pat. No. 6,230,890 describes a method where a mass of adhesive is first cooled, and thereafter packaged in a polymeric shrink wrap film.
In some processes, the adhesive mass is first pumped into a mold and then packaged. For example, U.S. Pat. No. 5,806,285 to Rizzieri teaches a method wherein adhesive is cast in a mold to form blocks. The mold has a plurality of holes formed therein and is lined with a thin film of plastic material which is vacuum thermoformed onto the inner surface of the mold. After filling the mold with adhesive, the open top surface is covered with a thin film of plastic material which is heat sealed to the film lining the interior of the mold. The mold containing the adhesive which is now enveloped by the film is then air cooled prior to removing the packaged adhesive from the mold.
Another process using molds is taught in U.S. Pat. No. 5,401,455 to Hatfield et al. The Hatfield et al patent describes a method for packaging hot melt adhesive compositions using a mold in the form of a pan lined with a film material which has its outer surface in contact with a refrigerant gas or liquid heat sink. Hatfield et al teaches that when molten hot melt adhesive is poured into the lined pan, the adhesive is fused to some degree with the film. According to Hatfield et al this in turn improves later mixing of the film with the adhesive.
Yet another process utilizing a mold is disclosed in U.S. Pat. No. 5,715,654 to Taylor et al. In this process, Taylor et al teaches lining a rigid mold with a thermoplastic film which is vacuum formed into the mold.
Still another process is described in U.S. Pat. No. 4,039,485 and involves the coextrusion of a sheath or coating surrounding a hot melt adhesive where the coextruded sheath material may be polyethylene.
Various other processes for packaging hot melt adhesives are illustrated in U.S. Pat. Nos. 5,373,682, 5,401,455, 6,155,029, 6,138,441, 5,669,207 and 5,942,082.
In all of the references cited above, the material used to package the HMPSA or other plastic material is a high molecular weight polymer, a modified high molecular weight polymer, or a combination of high molecular weight polymers. Typical examples are high molecular weight ethylene vinyl acetate (EVA) copolymer, or high molecular weight ethylene-acrylate copolymer, or high molecular weight low-density polyethylene, or a high molecular weight metallocene or single-site or Ziegler-Natta copolymer of ethylene. While these materials appear to be somewhat effective for their intended purposes, they also have readily apparent shortcomings which have detracted from their usefulness. These compositions, once melted, have a propensity under certain manufacturing conditions to not homogeneously mix with the hot melt adhesive which was enclosed therein because of the high viscosity of the polymeric material. As a result, these high molecular weight materials may lead to dephasing from the adhesive and therefore form lumps of crosslinked or gelled particles or char, which can clog filters and nozzles in conventional production machinery. Further, experience has shown that the addition of even a few percent, by weight, of these above-identified high molecular weight compositions to a typical hot melt or pressure sensitive adhesive composition has an adverse effect on the ability of the hot melt adhesive to form an effective bond with assorted target substrates. None of the references cited above teach how to formulate a film or coating composition in order to prevent this kind of defect.
Knowing the above shortcomings it has also been suggested to package pressure sensitive hot melt adhesives by formulating a film or a coating composition that is more easily meltable together with and/or more easily blendable into the molten adhesive composition itself just prior to application. The following illustrate some examples of this approach.
In EP 0957029, a coextrusion system is described where a tube of thermoplastic material is extruded and surrounded with a coextruded molten film which is pinched at regular intervals to create individual packages. The coextruded outer film is merely described as being a “non-adhesive” but it is also stated that the film composition may contain 1-5% by weight of the adhesive product.
Another illustration of this approach is shown in U.S. Pat. No. 5,865,927 where the packaging process involves: a) extruding hot melt adhesive through a die, b) spraying the surface of the extruded adhesive with a molten film forming polymeric material, the material being selected so that it will not detract from the properties of the adhesive composition when remelted therewith, c) heating the surface of the coated adhesive at a temperature and for a period of time sufficient to re-melt the film forming polymer so as to form a continuous coating thereof yet insufficient to melt the adhesive, and d) cooling the thus coated adhesive mass to a temperature suitable for handling. The film composition described in the '927 patent uses an EVA copolymer or a SEBS block-copolymer, an aromatic hydrocarbon resin, a paraffin wax, and an antioxidant/stabilizer.
Another example is described in U.S. Pat. Nos. 5,112,552, and 5,292,468, where hot melt adhesive compositions are poured into a mold in the form of a lined pan. The lining is sprayed onto the interior surface of the mold. Examples of the film composition used include an EVA copolymer, polyethylene copolymers, a paraffin wax, waxy forms of antioxidants, ethylene maleic anhydride, ethylene acrylic acid and natural rubbers.
EP 0557573 discloses a packaging composition for cold-flowing hot melt adhesive compositions. The packaging composition contains a blend of styrene-isoprene-styrene block copolymer, an aromatic hydrocarbon resin, mineral oil, a wax, and an antioxidant/stabilizer.
U.S. Pat. Nos. 4,748,796 and 4,755,245 disclose forming a protective coating for an adhesive by electrostatically coating a mold or cavity with a powder screen and then pouring molten hot melt adhesive into the mold. Powder materials are described as made of a wax, or as made of a polymer, or as made of a hot melt formulation, with no further specific formulations described.
Other approaches deal with pellet coating, like in U.S. Pat. No. 6,120,899, where a hot melt composition may be used to coat the pellets. The hot melt composition is described as containing a polymer, tackifying resin, and a small amount of wax. In U.S. Pat. No. 6,238,732, a pelletizing aid containing less than 10 wt % wax is used to coat adhesive pellets. In addition, several underwater pelletizing processes which form pellets of soft or tacky plastic products, like in U.S. Pat. No. 5,041,251, use anti-blocking agents that are applied either by extruding the pellets into a liquid emulsion containing the anti-blocking agent, or applied to the surface of the extruded pellets after the pellets are dried as solid particles. The anti-blocking agent is commonly a mineral powder or micronized wax powder or micronized polymer powder.
In some packaging processes, regardless of the form of the pieces of tacky or soft plastic material to be packaged, the material may be coated with wax, or with low molecular weight ethylene based polymer. In U.S. Pat. Nos. 5,942,304 and 5,733,645, polyethylene wax is used to improve the cuttability of soft and tacky materials. Materials used to form a coating under these conditions, however, typically cannot be formed into a continuous film, substantially surrounding the entire surface of the adhesive or soft plastic material pieces.
In the references cited above, if any benefit can be seen from using a lower-molecular-weight film composition or using a film material, which might include a wax, there are also apparent shortcomings which have detracted from their usefulness. First, wax or wax-based materials are usually too brittle to be formed into continuous films and/or to be used as packaging films as they can not be substantially stretched, folded or pinched without creating cracks and/or breaks therein which would lead to the adhesive or soft or tacky product's leakage from inside the package. This cracking and/or breaking behavior can also happen due to the intrinsic cold flow and subsequent deformation of the adhesive or plastic material during storage or shipment.
Further, none of the formulations containing a wax or lower-molecular-weight compounds described as a protective film in the references cited above would withstand severe shipment and storage conditions like what would be experienced under stacking pressure in a container on a truck, on a train or in a boat, in industrial regions where sunny weather conditions can be around 40° C. to 50° C., even reaching about 60° C. as a peak temperature during several hours. In these conditions, a thermoplastic film would typically have a tendency to get softer, to eventually melt, to become potentially tacky or become progressively plasticized due to the presence of a low molecular weight compound in the packaged material. This is a common failure, particularly in the adhesive industry that needs to be avoided in order to properly handle and use the packaged plastic material. None of the references exhibiting a lower-molecular-weight film or coating composition in the state of the art teaches how to withstand a migration test for a 3 mil thick film exposed during more than 3 days at 60° C., as set forth hereinafter.
Therefore, it has long been known that it would be desirable to have an improved packaging film for enclosing pressure sensitive, cold-flowing, hot melt adhesive compositions or tacky or soft plastic materials. The film material should advantageously be operable to reduce the attendant waste produced as a result of utilizing conventional packaging techniques, and which further reduces or substantially eliminates any troublesome clogging or other deleterious effects which may occur when it is used in combination with conventional adhesive application machinery. Further, the packaging film should have no substantial effect on the ability of the hot melt adhesive composition to form effective bonds on the desired substrates, and should not block to similar films at elevated storage and shipment temperatures. The packaging film should also be inexpensive to manufacture and sell, and further be characterized by its ease of utilization.